In observing and adjusting the track of a wheeled vehicle such as an automobile or truck, a source of a beam of light or light projector is mounted on one wheel and a scale on the other wheel on the same side of the vehicle. "Track" refers to the ability of the rear wheels to follow the path set by the front wheels. The scale bears graduations which are impinged by the projected light, signifying to the mechanic the extent to which the wheels are in track. Adjustments to the steering or suspension linkage can be made to adjust the track to be within acceptable limits for the vehicle.
Prior art scales are attached to structure that is mountable on the hub of the wheel or on the tread surface of the tire carried by the wheel. There are disadvantages of both. When the scale is attached to the hub, measurements from a reference point of the vehicle, such as the body frame, drive shaft or side wall, to the scale cannot readily be made. Such measurements are also difficult to make with respect to a scale mounted on the tread surface. Certain such prior art devices contact the outer side wall of the tire at its bottom, so that the scale is not oriented horizontally.